Small‐scale spatial variation in near‐surface turbidites around the JFAST site near the Japan Trench

Abstract

This paper aims to improve our understanding of the depositional processes associated with turbidites related to recent earthquake events. A series of short sediment cores (ca. 20–30 cm long) were recovered from the landward slope of the Japan Trench around JFAST (Japan Trench Fast Drilling Project) site C0019 by a remotely operated vehicle, KAIKO 7000 II, and the sample sites were accurately located using an LBL (long base line) acoustic navigation system. The properties of the cores were analyzed using visual observations, soft X‐ray radiographs, smear slides, measurement of anisotropy of magnetic susceptibility, and analysis of radioactive elements (134Cs, 137Cs, and excess 210Pb). For the first time, small‐scale (ca. 200–1000 m) spatial variations in recent earthquake‐triggered deep‐sea turbidites, the formation of which was probably linked to the 2011 Tohoku‐oki earthquake, are described. We also examine the submarine landslide that probably generated the sediment unit below the turbidites, which is thought to be an important process in the study area. The spatial distribution and characteristics of the near‐surface seismoturbidite obtained immediately after the earthquake, presented here, will enable precise calibration of offshore evidence of recent earthquakes, and thus facilitate the use of the sedimentary archive for paleoseismic interpretations. Furthermore, although sampling for turbidite seismology on steep slopes has not been widely performed previously, our results suggest that the recent event deposits may be continuously tracked from the slope to the basin using a combination of the present sampling method and conventional large‐scale investigation techniques.

Expedition 386 Scientific Prospectus: Japan Trench Paleoseismology

“…Also shown are previous IODP/ODP/DSDP drilling sites in the region and sites where surface cores and/or conventional (as long as 10 m) gravity and piston cores were retrieved after the 2011 earthquake. Solid red symbols = core locations with water depth >100 m where recent publications (Arai et al, 2013;Ikehara et al, 2014Ikehara et al, , 2016Ikehara et al, , 2018Kioka et al, 2019, submitted;McHugh et al, 2016;Noguchi et al, 2012;Nomaki et al, 2016;Oguri et al, 2013;Strasser et al, 2013;Usami et al, 2017Usami et al, , 2018Yoshikawa et al, 2016) or preliminary results from the most recent Sonne Cruise SO-251 (Strasser et al, 2017) document various distinct earthquake-related event deposits or stratigraphic gaps (Molenaar et al, 2019) Figure F2. Stratigraphic correlation between cores from isolated trench-fill basins in the central part of the Japan Trench (between 37°40′N and 38°10′N; alongtrench axis distance = ~50 km).…”

“…Correlation of the event deposit to the 2011 earthquake has been proved positive by short-lived radionuclide (Oguri et al, 2013;McHugh et al, 2016;Ikehara et al, 2016;Kioka et al, 2019) and transient disequilibrium pore water profiles (Strasser et al, 2013). Although there is local variability in deposition and preservation of the event layers (Yoshikawa et al, 2016;Ikehara et al, 2018), the general pattern obtained from >70 cores revealed a good along-strike correlation of documented event deposit occurrence within the reconstructed rupture area (McHugh et al, 2016, Ikehara et al, 2018Kioka et al, 2019). Extensive research is still ongoing to further calibrate the sedimentary record of the 2011 Tohoku-Oki earthquake and assess what earthquake parameters can reliably be deduced from the geological record, making the Japan Trench among the best study areas for calibration of submarine paleoseismology.…”

Expedition 386 Preliminary Report: Japan Trench Paleoseismology

“…Also shown are previous IODP/Integrated Ocean Drilling Progam/ODP/DSDP drilling sites in the region and sites where surface cores and/or conventional (as long as 10 m) gravity and piston cores were retrieved between the 2011 earthquake and 2019. Solid red symbols = core locations with water depth >100 m where recent publications (Arai et al, 2013;Ikehara et al, 2014Ikehara et al, , 2016Ikehara et al, , 2018Kioka et al, 2019aKioka et al, , 2019bMcHugh et al, 2016;Noguchi et al, 2012;Nomaki et al, 2016;Oguri et al 2013;Strasser et al, 2013;Usami et al, 2017Usami et al, , 2018Yoshikawa et al, 2016) or preliminary results from the most recent Sonne Cruise SO-251 (Strasser et al, 2017) document various distinct earthquake-related event deposits or stratigraphic gaps (Molenaar et al, 2019) linked to the 2011 earthquake and tsunami; open red symbols = locations where core data reveal no indication for recent sediment deposition (or erosion) related to the 2011 earthquake and tsunami as documented by Fink et al (2014), Ikehara et al (2016), Yoshikawa et al (2016), and Kioka et al (2019b); black symbols = location of cores for which no detailed information is available; cross symbols = sediment-trapped ocean-bottom seismometers (OBSs) or buried/displaced ocean-bottom pressure recorders (OBPs) associated with the 2011 earthquake (Arai et al, 2013;Miura et al, 2014). JMA = Japan Meteorological Agency.…”

“…Correlation of the event deposit to the 2011 earthquake has been proved positive by short-lived radionuclide (Oguri et al, 2013;McHugh et al, 2016McHugh et al, , 2020Ikehara et al, 2016Ikehara et al, , 2021Kioka et al, 2019b) and transient disequilibrium pore water profiles (Strasser et al, 2013). Although there is local variability in deposition and preservation of the event layers (Yoshikawa et al, 2016;Ikehara et al, 2018Ikehara et al, , 2021, the general pattern obtained from >70 cores revealed a good along-strike correlation of documented event deposit occurrence within the reconstructed rupture area (McHugh et al, 2016;Ikehara et al, 2018;Kioka et al, 2019b). Extensive research is still ongoing to further calibrate the sedimentary record of the 2011 Tohoku-Oki earthquake and assess what earthquake parameters can reliably be deduced from the geological record, making the Japan Trench among the best study areas for calibration of submarine paleoseismology.…”